1. Field of the Invention
The present invention relates to processes for the chemical-mechanical polishing of substrates, especially those comprising an insulator, a metal and titanium. More especially it relates to the polishing of substrates comprising silicon dioxide, tungsten, and titanium.
2. Description of Related Art
The chemical-mechanical polishing (CMP) of metals is common in the manufacture of semiconductor wafers. Vias are etched in silicon wafers and the wafer surface is then coated with the metal to fill the vias. This surface is then planarized by CMP to provide a perfectly flat surface for further processing of the wafer. Often the metal of choice for the circuit structure thus formed is tungsten. As shown in FIG. 1, often titanium 22 is coated onto the insulating base of silicon oxide 21 to provide a barrier to diffusion of tungsten 24 into the insulating layer and then a layer titanium nitride 23 is used to promote adhesion of tungsten 24 to the structure. Ideally, this structure is planarized to expose the insulating base before proceeding with subsequent steps in the manufacture of the integrated circuit. Rutten et al. ("Pattern Density Effects in Tungsten CMP", Jun. 27-29, 1995 VMIC Conference, ISMIC--104/95/0491) discusses the problems encountered when planarizing a structure in which tungsten, titanium and titanium nitride are used. Ideally, the titanium and titanium nitride layers should be removed at a rate comparable to the rate for tungsten removal.
In order to efficiently remove a metal layer, the surface of the metal is oxidized and the oxide is then removed by CMP. An iodate, such as potassium iodate has become a common oxidant for use in tungsten planarization. It has replaced the earlier use of hydrogen peroxide, H.sub.2 O.sub.2, as the oxidant because although H.sub.2 O.sub.2 provides a high removal rate for both tungsten and titanium, the removal from the center portion of tungsten in a via may be so great that dishing occurs as shown in FIG. 1b. Also, a void created in the metal layer during deposition may be exposed as shown in FIG. 1c creating a "keyhole" in the metal structure. This is particularly undesirable.
Also, in the planarization of the metal and titanium layers the rate of removal of the insulating layer, silicon dioxide, should be as low as possible. Often compounds which suppress the rate of removal of silica are included in the polishing slurries. Such compounds are described by Brancaleoni et al. in U.S. Pat. No. 5,391,258 and U.S. Pat. No. 5,476,606.